Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission of a vehicle improves power delivery performance and reduces fuel consumption by achieving nine forward speed stages and widening gear ratio span using a minimum number of constituent elements and securing linearity of step ratios.
Description of Related Art
In recent years, a rise in oil price causes dashing into unlimited competition for enhancing fuel efficiency.
As a result, researches into reduction of a weight and the enhancement of the fuel efficiency through down sizing are conducted in the case of an engine and researches for simultaneously securing operability and fuel efficiency competitiveness through multiple speed stages are conducted in the case of an automatic transmission.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components may be important in order to increase a fuel efficiency enhancement effect through the multistages.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, improvement of fuel economy may not be great.
In addition, if 8-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.